CSS blend modes offer web designers the ability to create visually appealing effects by blending layers such as background images and colors directly in CSS, without needing external editing software. The two key properties, background-blend-mode and mix-blend-mode, allow for various blending modes like multiply, screen, or overlay to enhance contrast and readability on web pages. These properties are widely supported across modern browsers, although older versions may lack full compatibility. Blend modes are leveraged for improving user interfaces, separating content from backgrounds, and creating dynamic visual effects, and they can be applied to various elements including images, text, and SVGs. While both properties allow blending, background-blend-mode focuses on blending within a single element's backgrounds, whereas mix-blend-mode blends an element's content with its parent's background. Despite their shared functionality, they are used in different contexts to achieve specific design objectives, allowing for enhanced creativity and interaction in web design.